In window shades that are commonly used in modern motor vehicles, a spring drive is used for tensioning the winding shaft of the window shade in the wind up direction of the strip shaped shade. The spring drive is formed by a helical spring that is accommodated in the interior of the winding shaft. This arrangement was predominantly utilized in the past because the winding shaft can be easily utilized as the housing for the spring drive due to its tubular shape. This was based on the notion that the spring drive can be accommodated in a space saving fashion at this location without creating major problems.
The winding shaft needs to carry out approximately 10-15 revolutions between a fully extracted position of the strip shaped shade and a completely retracted position in which the strip shaped shade is largely wound up on the winding shaft. As will be appreciated, these 10-15 revolutions of the winding shaft need to take place within the medium tensioning range of the spring drive in order to achieve a reasonable tension in both end positions. If the 10-15 revolutions were to take place at the beginning of the tensioning range of the spring drive, the tension would be too low in the wound up state. If they were to take place at the end of the tensioning range, the tension would be too high in the fully extracted position of the strip shaped shade. Consequently, the spring drive also needs to be tensioned relative to the completely relieved state of the spring drive by several revolutions, e.g., approximately 5-6 revolutions, when the strip shaped shade is completely wound up. Conversely, the spring drive requires a certain reserve, by which it could theoretically be additionally wound up, at the end of the wind up process. These requirements are associated with a specially designed spring drive in the form of a helical spring.
Another disadvantage resulting from arranging the helical spring within the winding shaft is the spring length required for achieving the above described marginal conditions. The length of the helical spring increases the risk of rattling, i.e., the spring windings hit the interior of the winding shaft because vehicle vibrations cause the helical spring to oscillate transverse to the longitudinal axis of the winding shaft. To prevent the spring windings from hitting the interior of the winding shaft, a common practice is to fill the interior of the winding shaft with a large portion of grease that serves as a damping material.
A coil spring generally has more favorable characteristics. When utilizing a coil spring, it is only necessary to maintain a certain tension at the beginning of the displacement path. The tension only increases in an abrupt manner immediately before the end of the displacement path is reached. Up to this point, the torque of a spring drive with a coil spring increases gradually in a relatively linear fashion.
Tensioning a cargo area cover for a motor vehicle with the aid of a spring drive is taught by DE 36 31 488 A1. This cargo area cover includes a housing defined by two end caps that partially surround the strip shaped cover wound on the winding shaft in a cup like fashion. The caps are provided with corresponding slots in order to extract the strip shaped cover. The wound up cover is exposed in its center region. One of the end caps contains a coil spring that is laterally adjacent the end face of the winding shaft. The inner spring end is anchored on a pin of the winding shaft. The outer spring end is fixed in a cavity of the end cap. The installation of the coil spring is relatively complicated because the cup shaped part of the spring housing is integral with the cup shaped part that represents the housing of the strip shaped cover. The DE 36 31 488 A1 reference does not provide any information as to the connection between the pin and the tubular winding shaft.